The ultrasonic flowmeter is characterized in that, for example, a structure is simple, there are a fewer number of mechanical movable parts, a measurable range of a flow volume is wide, and there is no pressure loss due to a flowmeter. In addition, the advances in the electronics technology in recent years has made it possible to improve measurement accuracy of the ultrasonic flowmeter. Therefore, researches for use of the ultrasonic flowmeter have been conducted in various fields, in which measurement of a flow volume of gas or fluid is necessary, such as a gas meter.
A structure and a measurement principle of a conventional ultrasonic flowmeter will be hereinafter explained. FIG. 25 is a block diagram showing an example of the conventional ultrasonic flowmeter. Ultrasonic transducers 1 and 2 are arranged across a flow path 12 in which fluid flows. The ultrasonic transducers 1 and 2 function as a transmitter and a receiver, respectively. More specifically, in the case in which the ultrasonic transducer 1 is used as a transmitter, the ultrasonic transducer 2 is used as a receiver. In the case in which the ultrasonic transducer 2 is used as a transmitter, the ultrasonic transducer 1 is used as a receiver. As shown in FIG. 25, a propagation path for ultrasonic waves, which is formed between the ultrasonic transducers 1 and 2, is inclined by an angle θ with respect to a direction in which the fluid flows.
When an ultrasonic wave is propagated from the ultrasonic transducer 1 to the ultrasonic transducer 2, since the ultrasonic wave travels in a forward direction with respect to a flow of the fluid, a velocity thereof increases. On the other hand, when an ultrasonic wave is propagated from the ultrasonic transducer 2 to the ultrasonic transducer 1, since the ultrasonic wave travels in a reverse direction with respect to the flow of the fluid, the velocity thereof decreases. Therefore the velocity of the fluid can be calculated from a difference between a time in which the ultrasonic wave is propagated from the ultrasonic transducer 1 to the ultrasonic transducer 2 and a time in which the ultrasonic wave is propagated from the ultrasonic transducer 2 to the ultrasonic transducer 1. In addition, a flow volume can be calculated from a product of a cross section of the flow path 12 and the flow velocity.
As a specific method of calculating a flow volume of fluid in accordance with the above-mentioned principle, a measurement method according to the sing around method will be explained specifically.
As shown in FIG. 25, the ultrasonic flowmeter includes a transmission circuit 3 and a reception circuit 6, and the ultrasonic transducer 1 is selectively connected to one of the transmission circuit 3 and the reception circuit 6 by a switching unit 10. In this case, the ultrasonic transducer 2 is connected to the other of the transmission circuit 3 and the reception circuit 6 to which the ultrasonic transducer 1 is not connected.
When the transmission circuit 3 and the ultrasonic transducer 1 are connected, the transmission circuit 3 drives the ultrasonic transducer 1, and a generated ultrasonic wave reaches the ultrasonic transducer 2 across the flow of the liquid. The ultrasonic wave, which is received by the ultrasonic transducer 2, is converted into an electrical signal, and a received signal is amplified by the reception circuit 6. A level of the received signal is detected by a level detection circuit 5.
FIG. 26 indicates an example of zero-cross detection in the conventional ultrasonic flowmeter. A peak hold circuit 13 generates a peak hold signal 19 from a received signal 18. The level detection circuit 5 detects that the peak hold signal 19 has reached a predetermined level 36 and generates a detection signal 37. A zero-cross detection circuit 7 detects a zero-cross point immediately after the detection signal 37 is generated, and a zero-cross detection signal 38 is generated. The zero-cross point is a point where amplitude of a received signal changes from positive to negative or negative to positive. This zero-cross point is assumed to be time when the ultrasonic wave has reached in the ultrasonic transducer 2. A trigger signal is generated in a delay circuit 4 at timing delayed by a predetermined time on the basis of the zero-cross detection signal 38. It is judged in a repetition unit 8 whether the zero-cross detection is to be repeated, and if the zero-cross detection is to be repeated, the trigger signal is inputted to the transmission circuit 3. A time from the generation of the zero-cross detection signal 38 to the generation of the trigger signal is called a delay time.
The transmission circuit 3 drives the ultrasonic transducer 1 on the basis of the trigger signal to generate the next ultrasonic wave. Repetition of a loop of transmission—reception—amplification/delay—transmission of an ultrasonic wave in this way is referred to as sing around, and the number of times of a transmission/reception loop is referred to as the number of times of sing around.
In a timing circuit 9, a time required for repeating the transmission/reception loop for a predetermined number of times is measured, and a result of the measurement is sent to a flow volume calculation unit 11. Next, the switching circuit 10 is switched to use the ultrasonic transducer 2 as a transmitter and the ultrasonic transducer 1 is used as a receiver, and measurement is performed in the same manner.
A value calculated by subtracting a value, which is calculated by multiplying the delay time by the number of times of sing around, from the time measured by the above-mentioned method to obtain a difference and dividing the difference by the number of times of sing around is a propagation time of the ultrasonic wave.
It is assumed that a propagation time at the time when the ultrasonic transducer 1 is set on the transmission side is t1 and a propagation time at the time when the ultrasonic transducer 2 is set on the transmission side is t2. In addition, as shown in FIG. 25, it is assumed that a distance between the ultrasonic transducer 1 and the ultrasonic transducer 2 is L and a flow velocity of fluid and a velocity of sound of an ultrasonic wave are V and C, respectively. In this case, t1 and t2 are represented by the following formula (1).
                                                                        t1                =                                  L                                      C                    +                                          V                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                                              t2                =                                  L                                      C                    -                                          V                      ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                          }                            (        1        )            
From these formulas, the flow velocity V is represented by the following formula (2).
                    V        =                              L                          2              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                ⁢                      (                                          1                t1                            -                              1                t2                                      )                                              (        2        )            
If the flow velocity V of the fluid is calculated, a flow volume Q is calculated from a product of a cross section of a flow path 14 and the flow velocity V.
In the above-described ultrasonic flowmeter, depending upon a flow volume, amplitude of a received ultrasonic waveform may vary significantly between the case in which an ultrasonic wave is propagated in a direction from the ultrasonic transducer 1 to the ultrasonic transducer 2, which is a direction in which the ultrasonic wave travels in a forward direction with respect to a flow of fluid, and the case in which an ultrasonic wave is propagated in a direction from the ultrasonic transducer 2 to the ultrasonic transducer 1, which is a direction in which the ultrasonic wave travels in a reverse direction with respect to a flow of fluid. In a state in which the flow of the fluid is disrupted, the amplitude may fluctuate significantly during sing around. In such a case, a zero-cross point immediately after the peak hold signal 19 has reached the predetermined level 36 is not always a point that is a predetermined wave number after a received waveform, and a measurement error occurs.
In order to solve such a problem, Japanese Patent Application Laid-Open No. 2001-116599 discloses a technique for using a received signal detection reference signal, which changes with time, as the predetermined level 36 to perform zero-cross detection at a predetermined wave number position of a received waveform.
In the above-described laid-open patent application, in the case in which time when a received wave reaches is earlier or later than a reference, the received signal detection reference signal is generated assuming that amplitude of the received wave decreases. However, in the case in which a flow of fluid is disrupted, it is likely that a magnitude of the amplitude does not always change as assumed and a measurement error occurs.
In addition, in the ultrasonic flowmeter of the above-described laid-open patent application, a measurement error due to noise cannot be reduced. FIG. 27 schematically shows an ultrasonic received signal obtained by amplifying an ultrasonic wave, which is detected by an ultrasonic transducer, in a reception circuit. As shown in FIG. 27, usually, a noise 39 is superimposed on an ultrasonic received signal 18. In such a case, a zero-cross point 40 is generated by noise at a point before an original zero-cross point 41. In this case, when amplitude of the ultrasonic received signal 18 changes, an inclination of the ultrasonic received signal 18 crossing a zero point changes, whereby the zero-cross point 40 generated by the superimpose noise 39 also shifts. In such a case, it is likely that a wrong propagation time is measured.
Moreover, in the case in which an apparatus, which consumes fluid flowing in a pipe, is connected to the pipe to which the ultrasonic flowmeter is connected, ripple may occur in the fluid flowing in the pipe due to an operation of the apparatus. Ripple means a periodical fluctuation in a change in a flow velocity of the fluid. In such a case, even if the zero-cross detection is performed by detecting a predetermined wave number position of a received wave, influence due to the ripple cannot be eliminated, the flow velocity fluctuates due to the ripple, and a measurement result different from an actual flow volume is calculated.
Therefore, in the case in which a gas meter is manufactured using the conventional ultrasonic flowmeter, it is likely that a large number of errors are included in a measured gas flow volume. In addition to the likelihood that an error simply occurs in measurement, it is also likely that, if a gas leak detection function is added to the gas meter, reliability of the detection function is deteriorated. In particular, in the case of the gas meter using the ultrasonic flowmeter, unlike a conventional diaphragm gas meter, since a flow volume of fluid flowing back in a pipe is also detected, the gas meter is significantly affected by the ripple.